Radioactivity in interplanetary dust

Most of the radioactivity in interplanetary dust in the region of the Earth's orbit results from interactions with solar protons. The most abundant nuclides having half-lives longer than a few hours in a dust of chondritic composition are Co56, Fe55, Al26, Mn53, Co55, C14, Na22, and Co57. The measurement of ratios of dust radionuclides can provide a means of measuring various time parameters. Among these are: the shrinkage time of the helicentric dust orbits, which is caused by the Poynting-Robertson and corpuscular drag effects; the fall times of the particle masses which survive vaporization upon entry into the Earth's atmosphere; the atmospheric residence time of the fraction of the dust which is vaporized; and the ages of certain geological sediments. Sampling and measurement of the dust is difficult due to the necessarily large size of samples (⩾107 cm2 yr), associated dilution of the activity by stable isotopes, and the presence of several sources of interfering activity. The best samples at the present time appear to be cores from sea sediments and the great ice sheets.